Switching devices, in particular, low-voltage switching devices, can be used to switch the current paths between an electrical supply device and loads and therefore to switch their operating currents. Thus, the connected loads can be connected and disconnected safely by the switching device opening and closing current paths.
An electrical low-voltage switching device, such as a contactor, a circuit breaker or a compact starter, has one or more so-called main contacts for switching of the current paths, which main contacts may be controlled by one or more control measurements. In principle, the main contacts in this case include a moving contact link and fixed contact pieces, to which the loads and the supply device are connected. An appropriate connection or disconnection signal is passed to the control magnet in order to close and open the main contacts, in response to which the armatures of these control magnets act on the moving contact links such that the contact links carry out a relative movement with respect to the fixed contact pieces and either close or open the current paths to be switched.
In order to improve the contact between the contact pieces and the contact links, appropriately designed contact surfaces are provided at points at which the two touch one another. These contact surfaces are composed of materials such as silver alloys which are fitted both to the contact link and to the contact pieces at these points and have a specific thickness.
The materials on the contact surfaces are subject to wear during every switching process. Factors which can influence this wear are:                increasing contact erosion or contact wear as the number of connection and disconnection processes increases,        increasing deformation,        increasing contact corrosion as a result of arcing, or        environmental influences such as vapors or suspended particles, etc.In consequence, the operating currents are no longer safely switched and this can lead to current interruptions, contact heating or to contact welding.        
For example, the thickness of the materials applied to the contact surfaces will decrease in particular as the contact erosion increases. In consequence, the switching distance between the contact surfaces of the contact link and the contact pieces becomes longer and in the end this reduces the contact force on closing. In consequence, the contacts will no longer close correctly as the number of switching process increases. The current interruptions resulting from this or else increased connection bouncing can then lead to contact heating and thus to increased melting of the contact material, which can then in turn lead to welding of the contact surfaces of the main contacts.
If one main contact in the switching device is worn or even welded, then the switching device can no longer safely disconnect the load. In the case of a welded contact at least the current path with the welded main contact will still actually in consequence despite the disconnection signal carry current and be live, so that the load is not completely disconnected from the supply device. Since the load therefore remains in a non-safe state, the switching device represents a potential fault source.
In consequence, the protective function can be blocked, for example in the case of compact starters according to IEC 60 947-6-2, in which an additional protection mechanism acts on the same main contacts as the control magnet during normal switching.
Fault sources such as these must therefore be avoided for safe operation of switching devices and therefore for protection of the load and of the electrical installation.